The field of the present invention relates generally to data reading devices, and more particularly to a host interface driver useful in a bar code scanner or other data reading device capable of supporting multiple electronic data formats and/or interface types.
Data reading devices such as bar code scanners and other symbol readers are a popular means for data acquisition in computerized processing systems. These devices are used to optically capture bar code patterns or other symbols or information imprinted on different surfaces in order to transmit the information encoded in the bar code pattern or symbol to a host processing device. Data reading devices may be used in a stand alone configuration, or integrated into terminals.
Bar code scanners in particular are a well-known and common class of data reading device. Two major types of commonly used bar code scanners are flying spot laser scanners and image based bar code scanners. Flying spot laser bar code scanners generally obtain bar code information by sweeping a laser spot across the bar code. The laser spot may be generated from a light source inside an oscillating reflecting surface, typically a mirror. The light reflected from the bar code is collected by a photosensor, which outputs an analog waveform representing the relative spacing of the bars in the bar code. The analog signal may then be digitized and decoded into data representing the information encoded in the bar code.
Bar code scanning devices based on solid state image circuitry, such as charge coupled devices (CCDs), are also conventionally known. These types of bar code scanners are typically implemented using either a one-dimensional or two-dimensional imaging array of photosensors (or pixels) to capture the bar code. One-dimensional CCD readers capture a linear cross section of the bar code at once, producing an analog waveform whose amplitude represents the relative darkness and lightness of the bar code. Two-dimensional CCD readers capture an entire two-dimensional image at once. In either case, the waveform created by reading the bar code is typically digitized and decoded for processing by a host processing device.
Host processing devices perform a variety of functions using data retrieved by bar code scanning devices. One familiar function involves the use of a bar code scanner in conjunction with a point-of-sale register. In a typical application, the bar code symbol of an item to be purchased is scanned, the bar code symbol is digitized and decoded, and the resulting data is used as a key by the host processing device to retrieve the item's description and price from stored memory.
In order to properly communicate with the type of host processing device to which it is connected, bar code scanners commonly employ an appropriate host device interface adapted to the particular characteristics of the intended host device. Examples of some of these interfaces include RS-232, IBM 46XX, keyboard wedge, and others. To illustrate the functionality of a host device interface, the keyboard wedge interface is typically used to interface a bar code scanner with a personal computer. The protocol of the keyboard wedge interface is used to convert data retrieved from the bar code scanner into a signal in the same form as that produced by the computer's keyboard. The computer is then able to process the input signal in the same fashion as keyed input data.
In most cases, different host processing devices use different types of interfaces, thereby requiring various electrical and data formats and protocols. Bar code scanning devices compatible with only a single host device interface have limited functionality because they can only be used with host devices implementing that particular interface. Because of the diversity of interfaces used by different host devices, single interface support significantly reduces the usefulness of a bar code scanning device.
To address these shortcomings, various approaches have been proposed to provide multi-interface functionality within a single bar code scanning device. In U.S. Pat. No. 5,258,604, for example, a system is proposed which implements a bar code scanning device capable of being configured with any one of a plurality of removable/replaceable interface boards inserted in the device's handle, in order to allow the bar code scanner to be used with host processing devices using different input/output interfaces. Replacing the interface board, however, is a difficult task for the typical user, and generally requires the services of a skilled technician. Moreover, because the interface board must be removed and replaced whenever the bar code scanner is used with a host processing device having a different host interface, the removal and replacement of the interface board on an ongoing basis can be unwieldy, inefficient, and time consuming. Such a process may also damage the interface board due to electrostatic discharge (ESD) problems when the interface board is manually handled.
U.S. Pat. No. 5,905,249, discloses several systems, one of which comprises an interface between a data acquisition device (such as a bar code scanner) and a host processing device accomplished through the use of a selected one of a plurality of interface cables. The data acquisition device is provided with multiple interface capabilities. The interface cable functions as the host-specific link between the data acquisition device and the host processing device, whereby the cable itself functions to have the data acquisition device set its own interface or otherwise combines with the acquisition device in order to communicate properly with the host. Use of different interconnect cables to form the correct host interface is much easier and more efficient than swapping circuit boards in order to configure the scanner. Moreover, there may be size constraints on scanner circuit boards, a situation which limits the number and type of host interfaces that can be supported by a single device.
At the same time, the physical size of bar code scanners continues to decrease due to market demand, placing space constraints on scanner circuit boards for the electronic components needed to implement multiple interface solutions. With bar code scanners supporting as many as three or more interfaces, the limits of current circuit board technology are being reached, and the cost to implement such combinations is becoming economically unfeasible.
There are also electrical constraints associated with current multi-interface designs. A common host device interface protocol, RS-232C, is often used when bar code scanning devices are connected directly to personal computers. The components commonly used in implementing the RS-232C interface are electronically incompatible with the components of other interface protocols generally used with bar code scanners. This incompatibility requires the use of electromechanical relays on the circuit board or other means to isolate other interface circuitry from the bipolar drive signal generated by the RS-232C circuitry. Depending on the interface pairings, one or more of these added electromechanical relays must be employed to provide the necessary circuit isolation. The relays are large and expensive, using up a large portion of the available scanner circuit board space. While other electrical techniques exist that can provide similar circuit isolation, they generally require equal or greater circuit board space and are equivalent in cost to the electromechanical relays. It would therefore be advantageous to provide a data reading device such as a bar code scanner capable of flexibly interfacing with a variety of different host devices. It would further be advantageous to provide such a data reading device having interface circuitry which is relatively compact and compatible among various different interface protocols.